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Diffusion of Iodide in Compacted Bentonite

AECL Research, Whiteshell Laboratories, Pinawa, Manitoba ROE IL0, Canada

N. G. Sawatsky

Soil Science Dep., Univ. of Alberta, Edmonton, Alberta T6G 2G5, Canada

S. C. H. Cheung

Civil Engineering Dep., Concordia Univ., Montreal, Quebec H3G 1M8, Canada

* Corresponding author.

ABSTRACT

Diffusion coefficients, D, are critical parameters for predicting migration rates and fluxes of contaminants through dense bentonite-based barrier materials used in many waste containment strategies. Values of D were determined for I^- (¹²⁹I is a relatively long-lived radionuclide present in high-level nuclear fuel waste) in saturated bentonite using both transient and steady-state techniques. The bentonite was compacted to dry densities, _b, ranging from 0.9 to 1.6 Mg m^–3, and saturated with a synthetic groundwater solution having an ionic strength of 0.22. Two different D values were determined: an apparent diffusion coefficient, D_a, defined as D₀, and an effective diffusion coefficient, D_e, defined as D_on_e where D_o is the diffusion coefficient in pure bulk solution, the tortuosity factor, and n_e the fraction of the porosity of the saturated clay that is available for diffusion. The value of D_a decreased from 6 x 10^–10 m² s^–1 at _b 0.9 Mg m^–3 to 1 x 10^–10 m² s^–1 at 1.6 Mg m^–3. The decrease in D_a with increasing _b is attributed to a decrease in as _b increased. The effect of _b on D_e was even greater: D_e decreased from 6 x 10^–11 m² s^–1 at _b 0.9 Mg m^–3 to 3 x 10^–12 m² s^–1 at 1.6 Mg m^–3. In addition to tortuosity effects, this decrease is ascribed to a decrease in n_e with increasing _b. For I^-, n_e is generally less than n (the total solution-filled porosity of the clay) because of factors such as anion exclusion. Within the _b range examined, the migration time for ¹²⁹I to move through the barrier material can be increased somewhat by increasing _b; relative to the long half-life of ¹²⁹I, however, the increased migration time is not significant. On the other hand, there appears to be a critical _b value for this clay of 1.4 Mg m^–3, beyond which the flux of ¹²⁹I from the clay can be markedly decreased. This critical value is probably a function of the specific surface area of the clay inasmuch as it influences the magnitude of the anion-exclusion volume. By compacting a bentonitic barrier material to a density greater than the critical value, the potential hazard associated with the long-term disposal of nuclear fuel waste that contains ¹²⁹I can be decreased.

Received for publication September 30, 1991.